This invention relates to positive temperature coefficient (hereinafter "PTC") devices or circuit elements, especially to those that can be surface mounted.
PTC devices whose coefficient of resistance substantially increases when the temperature reaches a certain range are widely used to protect electronic devices from overcurrent. These devices are conventionally made from polymers into which a conductive material is mixed. The PTC device is formed by compression molding and crosslinking the polymers with radiation.
Such a device may not show constant resistance under normal working conditions. Therefore, as described, for example, in Japanese Laid-Open Patent Publications Nos. 95203/1980, 165203/1981 and 218117/1986, additional steps can be carried out during the manufacture of PTC devices to improve the constancy of their resistance. One such step is to raise the temperature of the PTC device above the melting point of its polymer base. This rise in temperature maintains the PTC device's resistance constant under normal working conditions.
According to Japanese Laid-Open Patent Publication No. 95203/1980, thermally treating a PTC device for 10-20 minutes at 150.degree.-200.degree. C. and thereafter cooling it at least twice causes the resistance of the conductive polymer to reach a stable value.
According to Japanese Laid-Open Patent Publication No. 165203/1981, a PTC device of constant resistance can be made by annealing for a sufficient length of time at either the melting point of the polymer or a higher temperature.
According to Japanese Laid-Open Patent Publication No. 218117/1986, a PTC device showing constant resistance under high voltage can be made by the successive steps of (1) crosslinking the polymer with ionizing radiation, (2) applying heat at a temperature above the melting point of the polymer, and (3) crosslinking again with radiation or other means.
One of the methods most commonly used to surface-mount an electronic part to a substrate is soldering. Soldering can be divided into two categories: dipping in a bath of solder and reflow soldering. Either method exposes the part to be soldered to high temperature. Therefore EIAJ Standard RCX-0102/102 requires parts that will be surface mounted to maintain their thermal durability under either of the following conditions: (1) for the dipping method, dipping for 5.+-.0.5 seconds at 260.degree. C..+-.0.5.degree. C.; or (2) for the vapor phase solder bath method, immersion for 30.+-.1 seconds after the temperature has reached 240.degree. C..+-.5.degree. C.
However, these standard temperatures are both above the melting point of high-density polyethylene (approximately 130.degree. C.), which is the polymer of choice for making a polymer-type PTC device. Hence the resistance at room temperature of a conventional PTC device formed by compression molding increases excessively because of the high temperature applied during surface mounting. Such an excessive resistance makes surface mounting of conventional PTC devices practically impossible.
The PTC devices disclosed in Japanese Laid-Open Patent Publication No. 95203/1980, produced by heating to a temperature above the melting point of the polymer, cannot be readily used to protect against overcurrents because of their high volume-resistivity. The PTC devices disclosed in Japanese Laid-Open Patent Publication No. 165203/1981 are expensive to produce because a minimum of three hours is necessary for annealing. Furthermore, the body of a PTC device is formed by extrusion, and annealing reduces its resistance.
The PTC devices disclosed in Japanese Laid-Open Patent Publication No. 218117/1986 are probably effective as overcurrent protection elements. However, there is no disclosure in that Patent Publication that these devices may be surface mounted, nor is any highest temperature specified for thermal treatment of PTC devices.